1. Field of the Invention
The present invention relates to a hermetically coated optical fiber with improved long-term fatigue characteristics and initial strength.
2. Description of the Related Art
As shown in FIG. 1, an optical fiber generally comprises a core 1 made of glass, a cladding 2 surrounding the core 1 and a resin coating layer 3 surrounding the cladding 2. In such structure, moisture and hydrogen atoms easily pass through the resin coating layer 3 and react with glass as time passes, whereby the optical fiber tends to be broken. To improve long-term fatigue characteristics of the optical fiber, a structure of FIG. 2 is proposed. This structure has, between the cladding 2 and the resin coating layer 3, a dense layer 4 through which moisture or the hydrogen molecule hardly passes and which is made of an inorganic material such as SiC, carbon, TiC, Ti.sub.3 N.sub.4 and Si.sub.3 N.sub.4 or a metal such as aluminum (hereinafter referred to as "hermetic layer"), and an optical fiber having the hermetic layer is called a hermetically coated optical fiber.
Among the hermetically coated optical fibers, one coated with carbon suffers less deterioration of transmission characteristics and has better fatigue characteristics (cf. U.S. Pat. No. 4,319,803).
However, the conventional hermetically optical fibers have relatively weak initial strength.